The "extreme volatility" that would follow a British vote to leave the European Union could put downward pressure on air fares and lead Ryanair to move some of its investments out of Britain, chief executive Michael O'Leary said on Friday.

"After 9/11, after every crisis Ryanair is selling cheaper fares, we keep people flying. So the fact is it would have a downward effect on our pricing for 6 to 12 months, but we will keep people flying," O'Leary, whose airline flies more passengers than any other in the United Kingdom, told reporters.

"The longer term effect though is we will invest less in the UK, we will certainly switch some of our existing UK investment into other European counties because we want to continue to invest in the European Union and it will be bad for air travel and British tourism."

Brexit could spark recession in Britain

Britain's possible departure from the European Union represents a "significant downside risk" and could potentially push the nation into recession, International Monetary Fund chief Christine Lagarde warned Friday.

"A technical recession is one of the probabilities in the downside scenario in case of a Leave vote," Lagarde told reporters following publication of the IMF's update on the British economy, mirroring comments made on Thursday by Bank of England governor Mark Carney.